1. Field of the Invention
This invention relates generally to digital-to-analog converter circuitry and more particularly to a digital-to-analog converter in integrated-injection-logic form and method therefor.
2. Description of the Prior Art
Integrated circuits which perform digital-to-analog conversion are well known in the art. Integrated injection logic circuitry is also well known in the art and is widely used for increasing the density of circuitry fabricated within a monolithic integrated circuit. It is known by those skilled in the art that higher density of circuitry results in smaller chip dimensions, thereby yielding a greater number of integrated circuits formed from each processed wafer.
In prior art digital-to-analog converter circuits, it has been common to use a plurality of current sources which are switched by digital binary-weighted input signals for providing an analog current to the output; often resistors are used to determine the current that each current source contributes to the output current. However, resistors often require large amounts of chip area in the fabrication of the circuit. Also in the past, each current source transistor was fabricated in a separate epitaxial region which was isolated from every other current source transistor epitaxial region, which again caused the chip area necessary to fabricate the circuit to be increased. Thus, it will be obvious to those skilled in the art that a digital-to-analog converter circuit having current source transistors which need not be epitaxially isolated from one another and which does not require the use of resistors to fix the current conducted by the current sources will result in a more compact circuit and thereby lower the cost of manufacturing such a circuit.
In prior art I.sup.2 L (integrated-injection-logic) circuits, a current is supplied to the injection bar which supplies bias current to each I.sup.2 L transistor. However, the operating parameters for the I.sup.2 L transistors vary with temperature and with processing from circuit to circuit. Thus, the current conducted by the transistors varies also. For example, one operating parameter which varies with temperature and processing is beta (.beta.) which equals the collector current of a transistor divided by its base or bias current. Thus, for a given bias current, the collector current varies with temperature and processing. As most I.sup.2 L circuits allow transistors to saturate when conductive, the variation in operating parameters is usually not critical. However, for those I.sup.2 L circuits in which transistors must generate a fixed collector current, as in a digital-to-analog converter, the variation in operating parameters causes undesired variation of the collector current.